More the merrier

THE rich diversity of species in the Amazon basin is mainly down to buried ridges that criss-cross the region, a new study suggests. The finding debunks a long-held theory that it's rivers that isolated groups of Amazonian animals and cranked up their evolution.

We know there are up to three times as many vertebrate species in any given area of the lowland Amazon forests than is average for the rest of the world. Why the diversity should be so high is a long-standing puzzle. According to one popular theory the network of Amazonian rivers and tributaries left land animals isolated in pockets where they rapidly diversified from each other. If this is true, animals on opposite sides of the river should be genetically distinct, and these differences should become exaggerated as the river mouth widens.

Evidence to support this idea has been sketchy. Some studies suggested that rivers are a ...

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